Field of the Invention
The present invention concerns support structures for supporting cylindrical superconducting magnets, for example as used in Magnetic Resonance Imaging (MRI) systems. Such magnets must be cooled to below the transition temperature of the superconducting wire used, for example to a temperature of approximately 4K, which may be achieved by use of liquid helium, which requires the superconducting electromagnet to be placed in a cryostat to isolate it from ambient temperature.
Description of the Prior Art
FIG. 1 shows an example conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet comprises a coil structure 10 within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K. Other arrangements are known. Notably, certain arrangements do not require the provision of a cryogen vessel 12 around the coil structure 10.
The present invention provides a support structure to support the coils of a superconducting magnet 10 such as discussed above, which allows concentricity of the coils to the support structure to be maintained at all temperatures, while allowing relative movement of the coils with respect to the support structure in radial and axial directions due to thermal mismatch between the coils and the support structure, and radial and axial expansion or contraction of the coils due to magnetic loading.
The prior art contains several examples of support structures which have addressed similar problems. In one such arrangement, coils are wound into cavities defined on a radially outer surface of a cylindrical former, for example of aluminium. The coils are then impregnated with a thermosetting resin. When cooled to operating temperature, the former tends to contract to a greater extent than the coils. This results in the coils hanging loose on the former—essentially touching the former only at one circumferential point—or the coils may be held onto the former with coil clamps, which contact outer radial surfaces of the coils at circumferential intervals, and may result in the radially inner surface of the coils not touching the former at all. With such solutions, it is difficult to predict to a high accuracy the location of the coil centre, when cold. Point loading or stress concentrations from the support structure to the coils occur in this conventional structure.
U.S. Pat. No. 4,896,128 and GB2489126 describe superconducting coil assemblies.